1. Field of the Invention
This invention generally relates to a piston assembly in a compressor, and more particularly, to a piston assembly having a piston valve plate offset from the bottom of the compressor head so as to define a cooling chamber between the piston valve plate and the compressor head for reducing the temperature of the piston sleeve and the cup seal of the piston to increase the operating life of the cup seal and the compressor.
2. Description of the Related Art
A compressor receives a supply of fluid, such as a liquid or gas, at a first pressure and increases the pressure of the fluid by forcing a given quantity of the received fluid from a first volume into a smaller second volume using a piston assembly. A typical piston assembly consists of a compressor head connected to a valve plate, a piston sleeve pressure seated with the valve plate by an o-ring, and a piston that travels inside the piston sleeve. Compression of the fluid is typically achieved when the piston moves upward during an upstroke, forcing a given quantity of fluid received in the piston sleeve during the downstroke into a smaller volume at the compressor head.
A cup seal, which extends from the midsection of the piston, frictionally engages the interior of the piston sleeve in order to provide a seal between the pressurized and non-pressurized sides of the piston. The cup seal is necessary to prevent fluid from escaping around the piston during the upstroke compression process. The cup seal flexes during the upstroke and downstroke of the piston and the frictional engagement creates wear along the cup seal. Furthermore, typically, the cup seal is manufactured from a flexible plastic material that is susceptible to wear from heat. For these reasons, the operating life of a compressor is often dictated by the useful life of the cup seal.
Heat is prevalent when compressing air. In a conventional compressor, the act of compression generates heat in the compressor head where the air is forced into a smaller space by the upstroke of the piston. This heat conducts from the compressor head to the piston sleeve via the valve plate. Heat then conducts from the piston sleeve to the cup seal, which further hastens failure of the flexible cup seal, limiting the life of the compressor. Reduction of the temperature of the cup seal extends its life, and ultimately extends the life of the compressor.
In a piston assembly design known as a hard joint assembly, the piston sleeve is seated directly into a groove in the valve plate, creating a metal-to-metal contact point between the piston sleeve and valve plate. Because, the valve plate also functions as the base of the compressor head forming an area in which the gas is compressed, the heat of compression in the compressor head is directly transferred to the cup seal through the piston sleeve from the metal-to-metal contact of the valve plate with the piston sleeve. While the hard joint assembly does have heat transfer disadvantages, an advantage of the hard joint assembly is the fixed clearance volume between the top of the piston and the valve plate when the piston is at top dead center. In this assembly, it is easy to control the clearance volume, and the repeatability of the compressor""s efficiency can be achieved by accurately controlling the height of the piston sleeve and the clearance volume. Thus, the known standard compressor piston assembly designs do not inhibit heat flow from the compressor head to the piston sleeve, and, hence, the cup seal, while providing for consistent compressor performance.
Accordingly, it is an object of the present invention to provide a piston assembly for use in a compressor that overcomes the shortcomings of piston assemblies in conventional compressors. More specifically, it is an object of the present invention to provide a piston assembly that reduces the temperature of the piston sleeve resulting from heat conduction from the heat of compression originating in the compressor head through the valve plate to the piston sleeve.
Furthermore, it is an object of the present invention to reduce the temperature of the piston sleeve by providing 1) a nonconductive air gap between the compressor head and the valve plate, 2) increased surface area around the compressor head and valve plate for increased convective cooling of these surfaces, and 3) a radiant heat barrier between the compressor head and the valve plate for radiating heat from the compressor head, valve plate, or connection assembly joining the two.
These objects are achieved according to one embodiment of the present invention by providing a piston assembly that includes a piston sleeve having a top portion, a valve plate supported by the top portion of the piston sleeve, and a compressor head having a compressor head plate offset from the valve plate. By offsetting the compressor head plate form the valve plate, a cooling chamber is defined between the compressor head plate and the valve plate. In a preferred embodiment, this cooling chamber is open to atmosphere to maximize the surface area for convection cooling of the compressor head plate and the valve plate, thereby minimizing the amount of heat transferred to the piston sleeve. A conduit, preferably defined by a thermo-insulating material, traverses the cooling chamber to communicate gas between the interior of the piston sleeve and the interior of the compressor head via openings in the valve plate and the compressor head plate.
Additionally, it is an object of the present invention to provide a durable piston assembly containing a hard joint between the valve plate and the piston sleeve for providing a compressor assembly having a fixed clearance therebetween.
These and other objects, features and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.